UI freezes during decoding

[jscottanderson]

I'm fetching data from Firestore, mapping the documents and decoding each of them using FirestoreDecoder. However, decoding the documents momentarily freezes the UI. Running the code on the background thread makes no difference. How can I prevent the UI from freezing during the decoding?

let collection = Firestore.firestore().collection("roll_groups")

collection.addSnapshotListener { (snapshot, error) in
    if let error = error {
        print("Error fetching roll groups: \(error.localizedDescription)")
    } else if let snapshot = snapshot {
        DispatchQueue.global(qos: .background).async {
            let rollGroups = snapshot.documents.map { doc -> RollGroup? in
                do {
                    let rollGroup = try FirestoreDecoder().decode(RollGroup.self, from: doc.data())
                    return rollGroup
                } catch {
                    print("Error decoding roll groups: \(error)")
                    return nil
                }
            }
            
            DispatchQueue.main.async {
                completion(rollGroups)
            }
        }
    }
}